Continued warmth west, cool and wet east

We're starting a very warm week in California, and conditions will be about as summer-like as they get in San Francisco.  As a strong ridge of high pressure strengthens over the West Coast, temperatures will warm and offshore winds will keep the marine layer out of the Bay Area.  For much of the week daily high temperatures in San Francisco will reach the low- to mid-80s, well above normal, and many locations in the region will likely set new daily record highs over this period.  The image on the right shows forecast highs for Friday, but similar temperatures can be expected Wednesday and Thursday as well.  The dry, offshore winds will also result in fire concerns, and the NWS has issued a Red Flag Warning for the North and East Bay hills.  Any unattended fires could quickly get out of control given the hot, dry, windy conditions and abnormally dry vegetation for this time of year.

In the central and eastern U.S., temperatures have finally warmed up to near normal after a cool March and most of April.  However, the warm up is short lived as anomalously deep upper level trough moves into the region.  The images on the right are forecast 500 mb heights, winds, and vorticity for Tuesday night and then Friday morning.  On Tuesday, a strong trough is noted over the western Plains region.  However, instead of progressing quickly eastward as would typically be expected, this trough closes off and detaches from the steering flow (or jet stream).  Without the upper level steering, this closed low will be left to drift across the central Plains and will only slowly move east.  The system will bring unseasonably cold temperatures, with highs only in the 40s for many locations where average highs are well into the 60s.  The air will be cold enough that some forecast models also show the potential for accumulating snow over parts of the Rockies followed by the Upper Midwest (see final image).  To put this system in some historical perspective, the Minneapolis-St. Paul area has not seen measurable snow in May since 1991.

Assessing Record High Temperatures

I came across an interesting piece of information from the National Weather Service office in Minneapolis-St. Paul (MSP), showing that a new record low of 21 was set on April 20.  A single record temperature, high or low, is not necessarily notable, but part of what makes this story interesting is that the new record low set on April 20 was the first new record low set temperature set in the Twin Cities since August 2004.  For some perspective, the frequency in the occurrence of new daily record temperatures varies from location to location.  For stations with a long record history, setting new record temperatures is relatively rare.  MSP has a long record history, with observations dating back to 1871.  So, we wouldn't necessarily expect the Twin Cities to experience record temperatures very frequently.

What is really astonishing about this simple record low temperature is that while looking through daily records at Minneapolis-St. Paul since 2004, I found nearly 40 daily record high temperatures (see table).  In a static climate scenario, one would expect a roughly equal number of daily record high and low temperatures to be set over a given time period.  The fact that MSP has set nearly 40 record highs for just one record low since August 2004 is statistically quite unlikely.  Many of these record highs occurred in March 2012, an anomalously warm month for the Midwest that I have discussed in previous entries.  Data in the table at right are from the University of Minnesota Climatology Working Group.



Setting a single record high or low temperature it not especially useful in itself for assessing climate trends.  However, a series of records, or a much greater number of records in one direction versus another, is quite telling, and an increased frequency of record highs is expected under our current climate warming scenario.  If we plotted all of the observed high temperatures for a given date at a given site, we would expect a normal, or Bell shaped, distribution, with the majority of days clustered around the daily mean temperature (blue series on chart).  Record temperatures would be represented by the tails of this curve and would occur infrequently.  If we shift the distribution just a few degrees to the right, as is expected under a climate warming scenario, record high temperatures under the old scenario become much more common (red series on chart).  The chart is purposefully exaggerated for effect, but the unfortunately, depicts the pattern of record temperatures over recent years at MSP quite well.

Spring 2012 vs. 2013

A lot of attention this spring in terms of weather and climate has been focused on the relatively cold weather observed in the central and southern United States.  This pattern started in February and has persisted right up to mid-April, with a broad region experiencing below to well-below normal temperatures and uncharacteristically heavy snowfall for this late in the season, especially when compared to previous years.  The map on the right shows the temperature departure from normal for March 2013.  Areas in green, blue, and purple reported below normal temperatures (most of the central and eastern U.S. with the exception of New England).  Some areas of the northern Plains were over 10 degrees below normal for the month.  Areas of the southwest and New England were slightly above normal for the month.

The current spate of cold weather can be partially attributed to the current phase of the North Atlantic Oscillation, or NAO.  The NAO is technically the difference in sea-level pressure between the Icelandic Low and Azores High (two semi-permanent weather systems over the North Atlantic).  When these two systems are relatively strong, the difference in pressure between the two is greater, and this pressure differential generates a strong west-to-east jet stream across the Atlantic (see image below-left, courtesy of NC State University).  When this jet is stronger than normal, it generally keeps cold Arctic air over or near the polar regions; this is called the positive phase of the NAO.  When the two systems are relatively weak, the pressure differential decreases, and in turn, so does the strength of the west-to-east jet stream.  This allows cold Arctic air to move southward into the United States and Europe, and is called the negative phase of the NAO.  The image below-right (NOAA-CDC) shows the state of the NAO so far for 2013.  Since February, the NAO has been in its negative phase, which fits with the colder than normal temperatures observed over the U.S. and Europe.  The red lines are model predictions for the NAO over the next few weeks, and show a dramatic change to the positive phase, so temperatures may finally warm up for the central and eastern U.S. later this month.

It was a cold March for much of the lower 48 to be sure, but it was far from record breaking cold compared to the historical record.  So, any claim that this colder weather is evidence against the existence of climate change is severely in error.  In fact, one need only look back to March 2012 when temperatures were indeed record breaking - but were much, much above normal.  The map on the right shows temperature departures from normal for March 2012.  Note the huge area of above normal readings in the same regions that currently are running below normal.  Much of the Midwest was 10 to even 15 degrees above normal for the month, which easily made March 2012 the warmest March on record for the U.S.  The NAO was in its positive phase at that time, which may explain a portion of the warmer weather.  However, the unparalleled warmth of March 2012 likely had some contribution from human forced climate change, as I talked about in a post last year.  This type of extreme weather is expected with climate change - record warm temperatures will become more common and more extreme (like March 2012), and while periods of below normal temperatures will still occur (like March 2013), they will be less common and less pronounced.

       

Winter 2012-13 Recap

Winter 2012-13 was really a tale of two seasons for northern California.  November and December
were wetter than normal, when an active Pacific jet stream brought series of moisture-rich systems into the region.  January through March were much different, when a persistent upper-level ridge of high pressure over California deflected incoming storm systems well north of the state.  As a result, San Francisco recorded its driest January to March period on record with only 2.31 inches of rain, or over 9 inches below normal.  This record is also impressive due to the fact that records in San Francisco date back to 1850.  The heavier precipitation in November and December was very promising given the preceding very dry winter of 2011-12; unfortunately, this winter too will go down in the record books as another dry one for California. 

High elevation snow in the Sierra has been equally disappointing this winter.  Late March to early April is normally the peak in snow depth for California's mountains.  However, the most recent data show that California's snow pack currently holds only about 50% of normal for this time of year.  This is extremely concerning for agricultural and ecological concerns in the state, not to mention the upcoming fire season, especially following the dry winter last year.  Leading seasonal climate indicators for California weather, such as ENSO, have generally been neutral this season, with no preference toward unusually wet or dry conditions.  It's also implausible to pin the two consecutive dry winters on climate change, as California has experienced its share of dry winters in the historical record.  Climate research shows varying results in regards to future precipitation amounts for California, but the general consensus is for fewer but stronger winter time storm systems, which would likely result in more winters at both ends of the precipitation spectrum - either very dry or very wet.

Since the vast majority of California's precipitation falls between November and March, the state stands little chance of making a dent this spring in the current precipitation deficit.  Last year, despite the lack of rainfall, we were still able to benefit on the coattails of the wet 2010-11 winter season.  This year, California has no such luxury.